It is known in the prior art to use signal bandpass filters in communication systems to pass a known frequency signal and to prevent the passage of extraneous signals. In fail-safe communication systems it is desired to set the system signal threshold sensitivity to a predetermined level in a fail-safe manner, which requires that the signal gain ahead of the threshold reference must be fail-safe and therefore must not increase due to any failure. Similarly, any signal loss ahead of the threshold reference must be fail-safe and therefore must not decrease due to any failure. The two more common filter failures that can result in the reduction of the signal insertion loss of a filter are a short circuit from input to output and an open circuit of the ground reference of the filter.
A train vehicle control signal, for example a vehicle speed demand, can be binary coded such that a binary `one` indication is at a first frequency and a binary `zero` indication is at a second frequency. Respective bandpass filters can be employed to pass these frequencies to remove noise and achieve the desired selectivity, as illustrated in U.S. Pat. No. 3,810,161 of A. P. Sahasrabudhe, U.S. Pat. No. 3,775,750 of D. H. Woods, and U.S. Pat. No. 3,562,712 of G. M. Thorne-Booth et al. A signal receiver carried by the vehicle is operative to select the desired speed signal from the track signal block occupied by the vehicle to result in energizing the propulsion motors to move the vehicle along the track at an actual speed in accordance with said desired speed. It is known to use a crystal filter as a signal bandpass filter but such a crystal filter has a limited reliability and is a very complicated device. It requires substantial tuning of the crystal in associated circuits for impedance matching, and is expensive to obtain. To provide a low frequency and fail-safe signal bandpass filter, it is desired that the signal from a signal source of necessity passes through the filter.
It is known in the prior art to provide a low frequency mechanical bandpass filter device having an input and output electrically wired through a first piezoelectric transducer coupled to a first resonant bar, with a mechanical coupling provided between the first resonant bar and the second resonant bar, and a second piezoelectric transducer device coupled to the second resonant bar. A common connection is made to each resonant bar and if that common connection should break, or a short occur through one of the piezoelectric transducers, then direct electrical coupling might happen to make the device no longer fail-safe, with the occurrence of such a failure mode operating to bypass the narrow band filter effect and turn the device into the equivalent of a substantially wideband connector wire.